1. Field of the Invention
The present invention relates generally to a rotary damper or rotary snubber for braking a body of rotation, and more particularly, relates to a rotary damper which can prevent the generation of a large impact and sound on the occasion of opening and shutting a cover or a door or the like by installing the device on the cover or door of a piece of mechanical apparatus.
2. Description of the Prior Art
Hitherto, the opening and shutting of a cover or a door of a piece of mechanical apparatus functioned by rotating and moving a body of rotation reciprocatively within a fixed angular range.
In the case where the closure of a vertical opening is rotated and moved in the vertically downward direction and the closing is performed by a free fall, the kinetic energy of the closure is increased as the distance from the axis of rotation to the center of gravity of the closure increases. Therefore, the potential for a large impact will be generated by the time the closure completely covers the opening. Accordingly, a mechanism for braking the rotary power of the closure has been developed by prior workers in the art wherein a rotary damper has been arranged at the rotary axis of the closure in order to absorb this impact.
One such prior art rotary damper is disclosed and described in U.S. Pat. No. 4,098,597. In this device, since an inner tube through which many holes are bored in the surrounding wall must be constructed in the inside of an outer tube in order to form an accurate gap in a concentric circle between both of the tubes, there are inherent design problems including a multiplicity of parts, complexity in construction, difficulty in fabrication, many obstacles, expensive cost and the like.
Another example of a prior art rotary damper construction is illustrated in FIG. 7. Plural vanes or disks 2 or the like are received in the casing 1 and the shaft of rotation 3 is connected with the said vanes 2 integrally, so that the vanes 2 are rotated when the shaft 3 rotates. Working oil 4 fills all cavities within the said casing 1, and when the vanes 2 are rotated by transmitting the rotary power of the above-mentioned closure to the shaft of rotation 3, a rotary damper will be constituted. The structure generates a braking force by the viscous resistance of the working oil 4 as it is stirred by the rotary action of the vanes 2 within the casing 1.
But, the prior art rotary damper of FIG. 7 is a device in which only a fixed braking torque is given to the rotating member, and the effect of the rotary damper could not be altered to compensate for the change in torque caused by the position of the closure as the closure is vertically moved. That is to say, the torque constant of the rotary damper is established to a set value of the closure time of the closure. When a free fall is performed from an upper state by releasing the closure, the closure will stop halfway because of the fact that the torque at this time will be smaller in the closure than the torque in the rotary damper. The torque value of the rotary damper is set to a smaller value than the torque at the closure when the closure is dropped freely. Accordingly, the said prior art rotary damper cannot brake the rotary energy during the entire closure time of the cover, and there may arise a considerable generated impact upon complete closure.